Vascular Endothelium
Endothelium is composed of a single layer of flattened transparent squamous cells, joined edge to edge in such a manner as to form a membrane of cells. This is found on the free surfaces of the serous membranes, as the lining membrane of the heart, blood vessels and lymphatics. Endothelial cells also line the surface of the brain and spinal cord, as well as the anterior surface of the eye. Endothelial tissue originates from the mesoderm of the embryo, while epithelium arises only from the ectoderm or endoderm (Gray, H. in: Gray's Anatomy. Pick and Howden, eds., pp. 1083 and 1154, Bounty Books, New York, 1977).
Vascular endothelium forms a continuous, single-cell-thick layer which lines the entire circulatory system. Despite its microscopic dimensions (often less than 1 micron in thickness), this living tissue is a multifunctional organ whose health is essential to normal vascular physiology and whose dysfunction can be a critical factor in the pathogenesis of vascular disease. Anatomically, the vascular endothelium forms the physical boundary separating the intravascular compartment from all of the tissues and organs of the body. Biologically, this interface supports a number of vital functions.
First and foremost, the vascular endothelium comprises a “container” for blood. As long as this cellular layer remains intact and is functioning normally, a non-thrombogenic surface is presented to the circulating blood, thus allowing it to remain fluid and perform its nutritive functions unimpeded by intravascular clotting. Physical disruption of the endothelial lining, even on a microscopic scale, elicits an immediate hemostatic response, involving localized activation of the coagulation cascade and the adherence and aggregation of platelets, an adaptive reaction that serves to limit blood loss at sites of injury. Conversely, acute or chronic impairment of the non-thrombogenic properties of the intact endothelial lining (a form of endothelial dysfunction, see below) can be an important predisposing factor for intravascular thrombosis.
Because of its unique anatomical location, the vascular endothelium also functions as a selectively permeable barrier. Macromolecules encountering various regional specializations of the endothelium, including cell surface glycocalyx, cell-cell junctional complexes, microvesicles, transcellular channels and subendothelial extracellular matrix, are enhanced or retarded in their movement from (or into) the intravascular space. Selectivity of this barrier function typically reflects the size and/or charge of the permeant molecule, but may also involve active metabolic processing on the part of the endothelial cell. Enhanced permeability to plasma macromolecules, such as albumin, is a hallmark of acute inflammation, and, in the case of lipoproteins, is an important part of atherosclerotic lesion development. Pathophysiologic stimuli, as well as therapeutic drugs, that can modulate this endothelial function thus have potential clinical relevance.
Another functionally important consequence of the location of the vascular endothelium is its ability to monitor, integrate and transduce blood-borne signals. Through expression of cell surface receptors for various cytokines (IL-1α,β, TNF-α, IFN-γ, TGF-β), growth factors and other hormones (e.g., basic FGF, VEGF/VPF, insulin and insulin-like growth factors), as well as bacterial products, (e.g., Gram-negative endotoxins such as lipopolysaccharides (LPS) and related binding proteins), and their intracellular coupling, via second messenger cascades, to the metabolic and transcriptional generation of other biological effector molecules, endothelial cells function as important tissue response regulators. At every site in the circulatory system they are sensing and responding to the local pathophysiological milieu, and can help propagate these responses transmurally, from the intimal lining into the walls of larger vessels (e.g., coronary arteries), or from the luminal surface of capillaries directly into the interstitium of adjacent tissues (e.g., myocardium). This sensing and transducing function extends beyond classical humoral stimuli to the biotransduction of distinct types of mechanical forces generated by pulsatile blood flow (e.g., fluid shear stresses, circumferential wall stress and transmural pressure).
Endothelium is capable of generating a diverse array of biologically active substances, including lipid mediators, cytokines, growth factors and other hormone-like substances, many of which serve as important biological effector molecules, influencing the behavior of multiple cells and tissues. Some act directly within their cell of origin in a so-called autocrine mode, whereas others act on adjacent cells (within the vessel wall or in the blood) in a paracrine mode. Still other endothelial-derived mediators, such as hematopoietic colony-stimulating factors (GM-CSF, M-CSF) are secreted into the circulation to act at a distance, analogous to classical hormones. In addition to being the source of cytokines, growth factors and hormones, the endothelium also is an important target of their actions. Indeed, the capacity for the endothelium to undergo, local or systemic, “activation” in response to such stimuli, with resultant dramatic changes in functional status, is an important aspect of its biology and pathobiology. First demonstrated in the case of MHC-II histocompatibility antigen upregulation by T-lymphocyte products, and then extended to the induction of procoagulant tissue factor activity and endothelial-leukocyte adhesion molecules by inflammatory cytokines and bacterial endotoxin, the phenomenon of “endothelial activation” has become an important paradigm for modulation of endothelial phenotype. It provides a conceptual model that encompasses both physiological adaptation and pathophysiological dysregulation.
Given its interface location, integrating and transducing capability, and the vast repertoire of its biologically active products, the endothelium plays a pivotal role in a series of pathophysiologic events. In each, endothelial-derived agonists and antagonists dynamically interact in the regulation of important processes that can have both local and systemic ramifications, such as hemostasis and thrombosis, vascular tone, vascular growth and remodeling, and inflammatory and immune reactions. At any given time, factors influencing the activation state or functional integrity of the endothelium determine the relative set-points of each of these balances. For example, the intact, unactivated vascular endothelial lining is non-thrombogenic, because the net activity of antithrombotic factors, such as prostacyclin, thrombomodulin, cell surface heparin-like glycosaminoglycans and ecto-ADPases, exceeds that of the various pro-thrombotic factors potentially also generated by the endothelium. The controlled expression of certain of these pro-thrombotic factors in response to local vascular trauma (e.g., thrombin-induced von Willebrand factor release) can function adaptively, as part of a response-to-injury reaction; conversely, decreased production of anti-thrombotic factors (e.g., prostacyclin, tissue plasminogen activator) may contribute to intravascular thrombosis and vital organ damage.
Similarly, imbalances in endothelial-derived smooth muscle relaxants versus endothelial-derived vasoconstrictors can influence local circulatory dynamics, as well as systemic blood pressure. The vascular endothelium is the source of some of the most potent naturally occurring vasoactive substances known, including nitric oxide and related substances (originally described as an EDRF, or “endothelium-dependent relaxing factor”, by Furchgott and Zawadski) and endothelin-1, a novel peptide that resembles the lethal toxin in the venom of certain vipers whose bite can induce coronary vasospasm. Other factors in this endothelial vasomotor balance include prostacyclin, angiotensin II (generated by angiotensin converting enzyme at the luminal interface) and platelet-derived growth factor. The latter can be generated by endothelial cells and, in addition to its mitogenic properties, also is a potent smooth muscle contractile agonist.
Under normal conditions, the cells of the vessel wall are essentially growth quiescent, but following experimental endothelial denudation, a burst of medial smooth muscle migration and division is triggered, which then subsides as endothelial regeneration occurs. This well orchestrated wound healing response presumably reflects not only the localized generation or release of growth stimulators but also a transient, relative deficiency in endothelial-derived growth inhibitors. The resultant intimal hyperplasia is very similar to that which occurs in early atherosclerotic lesions. The more complex issues of sustained smooth muscle hyperplasia, secondary to immune-mediated endothelial damage in transplant-associated arteriosclerosis, or in the post-angioplasty setting, as well as the interplay of angiogenic and anti-angiogenic factors in neovascularization phenomena in ischemic myocardium and peripheral tissues may also reflect imbalances in endothelial-derived growth regulators.
It is now increasingly clear that biomechanical stimuli derived from flowing blood can modulate the phenotype of endothelial cells. An important aspect of this phenomenon is the ability of these forces to alter the patterns of genes expressed by vascular endothelium. A growing body of in vitro experimental data has demonstrated that when cultured endothelial cells are subjected to defined biomechanical stimuli they can manifest alterations in gene expression. Interestingly, many of the genes that have been demonstrated to be regulated by these stimuli have been found to be expressed in vascular endothelium in vivo.
Vascular diseases including thrombotic complications are a major cause of death in the industrialized world. Examples of these complications include acute myocardial infarction, unstable angina, chronic stable angina, transient ischemic attacks, strokes, peripheral vascular disease, preeclampsia, deep venous thrombosis, embolism, disseminated intravascular coagulation and thrombotic cytopenic purpura, thrombotic disorders, inflammatory disorders, chronic vascular disease, autoimmune disorders, transplant vasculopathy/rejection, atherosclerosis, hypertension, aneurysmal disease, vasospastic syndromes, ischemic coronary syndromes, cerebral vascular disease, angiogenic (both pro and anti) processes, and wound healing. Thrombotic and restenotic complications also occur following invasive procedures, e.g., angioplasty, carotid endarterectomy, post CABG (coronary artery bypass graft) surgery, vascular graft surgery, stent placements and insertion of endovascular devices and prostheses.